/**
 * 
 */
package niuke.xiaohongshu;

import java.util.Scanner;

/**
 * @author zhongfang
 *
 */
public class t3 {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		long[] sum = new long[N];
		long s = 0;
		for (int i = 0; i < N; i++) {
			s += sc.nextLong();
			sum[i] = s;
		}
		int K = sc.nextInt();
		sc.close();
		long max = Long.MIN_VALUE;
		for (int i = 0; i < N - K; i++) {
			max = Math.max(sum[i + K] - sum[i], max);
		}
		System.out.println((double) max / K);
	}

}
